"When caught and tried for this heinous crime they were sentenced to 30 years each."
Bear in mind they were not just robbers - they were armed violent criminals who beat up the train driver and at least two other people. It actually was a heinous crime.
"This sentence, which was far more than usually given for murder, shocked the UK."
Murder was then, as now, a mandatory life sentence, with a recommendation for a minimum term. Thirty years, though significant, is less than life. And they all served far less than thirty years. And did the sentences really "shock the UK"? Any evidence for that?
"The result was that, knowing that the judges might be very harsh on robbers, the perpetrators started killing those police officers and members of the public trying to stop or arrest them."
Do you have any actual evidence for this remarkable assertion?